This invention relates to rotary regenerative heat exchangers and more particularly to detecting hot spots in rotary regenerative heat exchangers before the element ignites.
In a rotary regenerative heat exchanger, a mass of heat absorbent material commonly comprised of packed element plates is positioned in a hot exhaust gas passageway to absorb heat from the hot gasses passing therethrough. After the plates become heated by the gas they are positioned in a passageway being traversed by cool air where heat is transferred from the heated plates to the cool air or gas flowing therethrough.
The heat containing gasses are typically the exhaust gas from a combustion process. As the hot exhaust gasses are directed through the rotary regenerative heat exchanger, fly ash and unburned products of combustion carried by the exhaust gas are deposited on the surface of the packed element plates. The deposits continue to build up until air and gas flow through the heat exchanger is reduced at least in the region of the build-up. When the temperature is elevated to the ignition point of the deposit heat is then generated until the deposits begin to glow and cause a "hot spot", that if not detected will rapidly increase in temperature until the metal of the heat exchanger will itself ignite and cause a fire. U.S. Pat. Nos.: 3,730,259; 3,861,458; 4,022,270; and 4,383,572, the disclosure of each being hereby incorporated by reference, disclose apparatus to detect hot spots in the packed element plates of a rotary regenerative heat exchanger including multiple elements and the cleaning of the lens of each element.
Typical prior art hot spot detectors rely on a single pass of the hot spot within the rotary regenerative air heater past a sensor to detect a hot spot. Furthermore, the signal produced by each of the plurality of sensors has been combined and processed as a single signal to detect a hot spot. Prior art hot spot detection systems typically use a predetermined fixed temperature as the threshold above which a measured temperature is considered to be a hot spot.